Welcome to Flyertalk and welcome to the BA Forum. The main thread for this is the EC261 thread:
https://www.flyertalk.com/forum/brit...261-uk261.html
and this has a lot of information on this. That thread, in the wiki, has something about Mennens, the reimbursement (not compensation) route for downgrades, that's on top of the cancellation compensation, however it's pro-rate to the total distance and therefore may not work out as a significant some given the companion vouchers. Because it's a cancellation, compensation is likely to be payable, in a few days have a look here to find out more - it takes a few days for the information to come though. But I suspect it is clalmable for the delay. Make a careful note of timings, since this can be important.
https://www.flyertalk.com/forum/brit...3-edition.html
Then this thread tells you what if anything you can do about this situation:
https://www.flyertalk.com/forum/brit...need-know.html